The proposed 3D localization system, which is based on a single partially-connected R-RIS hardware architecture comprising subarrays of meta-atoms and autonomously estimates the position of mobile users lying within the surface's area of influence [1]}, is illustrated in Fig. REF .Although Fig. REF  depicts an R-RIS as a uniform planar array with regular partitioning comprising four subarrays, our proposed AoA-based 3D localization system requires, in principle, only three subarrays. The impact of the number of subarrays and that of the R-RIS partitioning on the localization accuracy of our localization scheme will be investigated in Section . It is noted that the proposed RIS framework comprising subarrays of meta-atoms could replace the RF front-end of a conventional multi-antenna BS, realizing a metasurface-based holographic RX [2]}, [3]} capable of MS localization through narrowband measurements. Following the metasurface hardware designs in [4]}, [2]}, [6]}, each subarray incorporates a waveguide that directs the impinging signals on its meta-atoms to a single output port feeding a reception RF chain. Note that such an RX RF chain consists of a low noise amplifier, a mixer that downconverts the signal from RF to baseband, and an analog-to-digital converter [7]}. The phase configurations of all subarray's elements result effectively in an analog beamformer/combiner realizing a spatial reception filter, which is applied on the impinging waveform. In this paper's framework, this feature implies the spatial sampling of the MS transmitted pilots intended for localization. We assume that the MS is located far from each R-RIS subarray. Without loss of generality, we further assume a single-antenna MS, and leave the multi-antenna case as well as the tracking problem for the case of mobile MSs for future works.
